This invention relates to steam generators and more particularly to systems for removing sludge deposits from the tube sheets of steam generators.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet header. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacemt the shell, and a feedwater ring disposed above the U-line curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is conducted down the annular chamber adjacent the shell until the tube sheet near the bottom of the annular chamber causes the feedwater to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the feedwater is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment generating electricity in a manner well known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tube walls which may be constructed from Inconel, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes. However, experience has shown that under certain conditions the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, a highly undesirable result.
There is now thought to be at least two causes of tube leaks in steam generators. One cause of these leaks is considered to be related to the chemical environment on the feedwater side of the tubes. Analysis of tube samples taken from operating steam generators which have experienced leaks has shown that the leaks were caused by cracks in the tubes resulting from intergranular corrosion. High caustic levels found in the vicinity of the cracks in the tube specimens taken from operating steam generators, and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions have identified high caustic levels as the cause of the intergranular corrosion and thus the cause of the tube cracking.
The other cause of tube leaks is thought to be tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on the tubes near the tube sheet at levels corresponding to the levels of sludge that has accumulated on the tube sheet. The sludge is mainly iron oxides and copper compounds along with traces of other metals that have settled out of the feedwater onto the tube sheet. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and tube wall thinning locations strongly suggests that the sludge deposits provide a site for concentration of the phosphate solution or other corrosive agents at the tube wall that results in tube thinning.
One known method for removal of this sludge is referred to as the sludge lance-suction method. Sludge lancing consists of using high pressure water to break up and slurry the sludge in conjunction with suction and filtration equipment that remove the water-sludge mixture for disposal or recirculation. In the sludge lancing method a six inch handhole is used to provide access so that two flexible perforated suction headers may be placed on and along the periphery of the tube sheet around the tube bundles. A high velocity water lance is then introduced through the handhole and aligned between the tube rows. The lance is then moved along the tube sheet while two high velocity water jets are established perpendicular to the movement of the lance. The water jets force the sludge toward the periphery of the tube sheet where the water-sludge mixture should be sucked into the flexible suction headers. While theoretically the sludge lance-suction method should remove the sludge deposits, experience has shown that it is not too effective.
One of the problems with the sludge lance-suction method is that a wide slot of water, caused by reflection or expanded water volume, exits the tube bundle near the periphery of the tube sheet and overwhelms the suction headers' capacity. Consequently, the sludge is either redeposited in the wrapper area or washed back into the tube sheet. Also when the second side of the generator is being lanced a significant amount of washback will occur to the side which was lanced first. In addition, the suction header required such an abundance of holes in the flexible headers that it was not possible to maintain a sufficient suction near the end of the header. Furthermore, it is not mechanically feasible to properly align the holes in a flexible header where access is as limited as in the case of a nuclear steam generator.